1. Field of the Invention
The invention generally relates to a converter-cooling element assembly, also referred to as converter-cooling element assembly below, and to a method for producing a converter-cooling element assembly. More particularly, the invention relates to a ceramic converter, which is joined to a cooling element by a metallic connection.
2. Description of Related Art
Fluorescence converters, commonly also referred to as converters, are employed for converting light (or electromagnetic radiation) of a first wavelength into light of a second wavelength.
To this end, the converter is excited by a primary radiation light source with a first wavelength. The light from the primary radiation light source is converted by the converter, at least partially, into a secondary radiation having a second wavelength. A portion of the introduced light energy is converted into heat within the converter. It has to be removed from the converter as efficiently as possible to prevent an excessive temperature increase in the converter which might otherwise lead to a destruction of the converter material, depending on the converter material used. Moreover, even in converters that have a comparatively high thermal destruction threshold, conversion efficiency decreases as the temperature increases. This effect is caused by so-called “thermal quenching”.
In order to minimize or avoid the adverse effects described above, devices for light conversion usually include a cooling element, for example in the form of a heat sink, by means of which the heat can be dissipated from the converter material. Decisive factors for efficient heat dissipation from the converter are in particular the thermal conductivity of the converter material and thermal conductivity of the connection between converter and cooling element.
WO 2009/115976 A1 describes a device for light conversion in which the converter is divided into individual sections. By dividing the converter material into smaller individual sections, rapid heat dissipation from the converter via the heat dissipating element to the heat sink is intended to be ensured. These individual converter sections are in contact with a thermally conductive and reflective material.
One option for thermal connection of converters to cooling elements described in the prior art comprises the use of an adhesive, for example an epoxy- or silicone-based glue.
For example, US 2012/0057364 A1 describes thermal connection of a converter by means of an adhesive which contains thermally conductive fillers.
However, the adhesive layer significantly contributes to the thermal resistance of the overall system thereby limiting the allowable power of the excitation light and hence also the light flux and luminance that can be achieved. Moreover, the glue connection is usually a limiting factor for the lifetime of the overall system.
Furthermore, the connection between converter and cooling element has to meet optical requirements, in particular sufficiently high reflection. The secondary light in the converter is emitted isotropically at any location, so that for example in a remission configuration light components of the secondary light will not be emitted in the useful direction but towards the cooling element. In order for these light components to be useful as well, they should be reflected by the cooling element.
This is achieved by employing a mirror in the prior art. This mirror is usually applied to the cooling element by means of a thin adhesive layer (e.g. based on a silicone or epoxy glue).